darcyw

Hi all
question- which O2 sensor gets the defouler on an OEM cat? Is it the one in the header BEFORE the cat, or the one IN the centre of the cat?

Much appreciated.

darcy

fernando.

Why do you need to run a defouler on an OEM setup?

It goes in the secondary oxygen sensor port - the one in the cat.

flanders

Is your cat not working or why do you want to use a defouler?
The defaulers are used on the secondary sensor and on the stock cat that sensor sits in the middle of the cat.
Edit: seems I was a bit slow

jwzimm

Why would you put a defouler on an OEM cat? You would only put one of these on an O2 sensor when you have no cat (or a gutted cat) on the car. If you had one of these you would put the defouler on the O2 sensor after (or inside) the CAT. You do not want to do this on the pre-CAT sensor as that would screw up your ECU.

darcyw

in reading a previous post from way back, o2 sensor tips that break off are trouble for the cat itself and necessitates cat replacement if it happens. the defouler keeps this from happening. i'm not throwing any codes, but like the extra insurance the defouler offers. i had already loosened off the cat o2 sensor but then saw the one in the header before the cat and thought...hmmm...maybe its that one instead.

darcy

s2000ellier

the o2 sensors don'tjust break, the honey comb in the cat breaks apart and takes out the sensor.

darcyw

oh. Now I know. I didn’t quite get it in my research...I found a nice stainless defouler from vibrant I couldn’t pass up.
darcy

jwzimm

That sensor is critical to the functionality of the engine. The ECU needs to know what the content of the exhaust stream contains in order to adjust the fuel trims. If you shroud it in a defouler you remove the sensing element from the exhaust stream. This will cause the sensor to give false or delayed readings which will cause your engine to run rich or lean.

Remove the defouler.

darcyw

Thanks for the input- its something I've been wrestling with and I completely understand the issues you bring up regarding running rich/lean. The defouler I have came with various sized orifices- which I did NOT use. I just left the defouler as is with no restriction. I recall reading from Billman that its not an issue for OEM cat to run with an open defouler as referenced below.

"A defouler will 100% keep the o2 sensor from being sheared off, and it will also HELP the CEL to stay off (it will prevent code P0420 from creeping up should the cat ever become inefficient). Adding a defouler to any S2000 with a stock cat is valuable insurance.
On the same token, if you gut the cat, use the defouler to keep the CEL off."

​​​​​​​darcy

jwzimm

He is referring to the efficiency sensor that is installed inside the CAT between the catalyst bricks, not the primary sensor installed before it. This is further proven by his statement regarding the prevention of the CEL for inefficiency. How would the primary sensor detect a failing CAT at all? It is in front (upstream) of the CAT and unable to detect anything about how the catalyst is operating.

Again, the defouler, even with any orifices removed, will take the sensor bulb out of the gas stream. This will impact the operation of the sensor and cause issues with the engine.

If you don't believe me, PM Billman and have him clarify for you. You will find he agrees with me.